Various methods have been developed for the qualitative inspection of the electronic integrity of a solder joint, including sonic and ultrasonic testing methods with or without an electric current load; thermal testing methods to determine the heating and cooling-off responses of a solder joint, and X-ray analysis of solder joints. These methods are slow, tedious, and have not received universal acceptance because of the lack of proven performance and of the testing equipment cost associated therewith.
Vanzetti (U.S. Pat. No. 4,481,418) has demonstrated that thermal information is useful for determining the electronic integrity of a solder joint. In the Vanzetti system, a laser beam sequentially illuminates and heats each solder joint on a circuit board individually. The energy imparted to a joint raises the joint's temperature, and an infrared sensor sequentially measures the rate at which the temperature rises and decays for each joint during the illumination thereof with the laser beam. In a bad joint (i.e., a joint in which the contact with the desired electrical conductors is poor) the initial temperature rises very high during illumination, then will decay more slowly than for a good joint following illumination. The Vanzetti technique has been shown in some cases to be a sensitive and reliable detector of deficient joints. However, the Vanzetti system requires that the laser heating source must be focused, and has to be mechanically moved from solder joint to solder joint to spot-heat each solder joint individually, with the attendant registration and programming problems. In addition, the Vanzetti system heats and reads each solder joint in a single pulse, and thus does not address a stabilized system. It does not permit repeated readings to improve the signal-to-noise ratio. Within an improved signal-to-noise ratio, smaller temperature swings can be measured to determine the electronic integrity of a particular joint. In addition, since the Vanzetti system relies on direct laser illumination of a joint to heat the joint, the technique is quite sensitive to surface contamination of the joint. This contamination can cause appreciable changes in the already low emissivity- and therefore absorption - of the joint to the laser radiation.
The present system overcomes many of the disadvantages of the existing systems because of its speed, reproducibility and flexibility in that existing testing equipment can quickly and inexpensively be modified to carry out the system of the present invention. The present system illuminates the entire board, which permits a more efficient coupling of heat into the solder joints via connecting conductors, and improves the signal-to-noise ratio because surface contamination effects are reduced. In addition, no focusing or mechanical motion of the heating source is required; a simple, fixed heating source can be used. Further, the present system uses repetitive heating pulses, permitting detection of oscillating joint temperature excursions, which permits the signal-to-noise ratio improvement.